For everyone that didn't know already how to spectate high elo games.

How to spectate high elo games?

1. Go to your summoner profile.
2. Go to 'Leagues' tab.
3. From up right, switch from 'MY LEAGUES' to 'CHALLENGER TIER'.
4. In Challenger leaderboards section, if there is an eye on someone's profile, right click it and then there should be available the option 'Spectate Game'.

Now you are watching an online challenger tier game. Just made a thread because I found it myself and wanted to help the people that were in the same situation as me 5 minutes ago who didn't know that it is possible to do this anymore (since the spectate mode is not in main menu page anymore).